1. Field of the Invention
The present invention relates to a symbol reading device such as a bar code reader or an optical character reader (OCR) which optically reads bar codes, characters and other symbols formed on the surface of various objects.
2. Description of the Related Art
Because of their ability to read characters and symbols on the surface of various objects without actual physical contact with those objects, bar code and optical character readers are enjoying extensive use. These symbol reading devices are used as both stationary and hand held symbol readers. It is desirable that the distance between a symbol or bar code carrying surface and the symbol reading device over which symbols or bar codes can be read (i.e., the reading distance) have a fairly broad range as defined by upper and lower limits (i.e., the reading range).
Conventional symbol reading devices scan symbols such as bar codes using a laser beam and receive reflected light from the symbol faces with a light receiving device. An output of the light receiving device represents the intensity of the reflected light. For example, a bar code consisting of black bars and white spaces will reflect light which causes the light receiving device to produce a small signal for bars and a large signal for spaces. Therefore, after suitable processing (i.e., amplification), the output of the light receiving device may be discriminated with reference to a proper slice level, thereby producing a binary signal in association with the scanned bar code. The symbol reading apparatus then identifies the scanned bar code based on the binary signal.
An He-Ne laser has commonly been used as a light source for generating a laser beam, but in recent symbol reading devices, the use of a semi-conductor laser has increased with a view to reducing the overall size and weight of the symbol reading device. However, laser light issuing from a semi-conductor laser is diffusive and usually requires focusing with a lens to produce the nearly parallel rays of a laser beam. Because a bar width of 0.2 mm and less is by no means rare in bar codes, the scanning laser beam must be focused to a spot diameter of 0.2 mm and less in order to enable identification of such small bar widths. Due to these circumstances, semiconductor laser beams are not completely collimated, but are convergent with a focal point at a specific distance.
High resolution symbol reading is possible near focal points since the diameter of the beam spot is quite small. However, at positions remote from the focal point only low resolution symbol reading can be achieved. Thus, conventional symbol reading devices employing a semi-conductive laser have unacceptably narrow reading ranges.
A first prior art technique directed to this problem is described in Unexamined Published Japanese Patent Applications No. 304589/1989 and No. 7182/1990. These documents propose the expansion of the reading range by mechanically moving the laser beam light source, lenses and other optical components to vary the optical path length of a scanning laser beam. Thus, the distance between the symbol reading device and the focal point location is variable.
The basic layout of the device disclosed in Unexamined Published Japanese Patent Application No. 7182/1990 is shown in FIGS. 1A and 1B. As shown, the light from a semi-conductor laser light source 201 is condensed by a condensing lens 202 to form a laser beam 203. The laser beam 203 has a beam waist BW in the focal position at distance FL which is determined by the relative positions of the semiconductor laser light source 201 and the condenser lens 202. If a symbol such as a bar code is read out at the position of the beam waist BW, reading with maximum resolution can be accomplished.
In the technique depicted in FIGS. 1A and 1B, the condenser lens 202 is displaceable in direction 204 along an optical axis of laser beam 203; wherein the distance FL from the semiconductor laser light source 1 to the focal position, beam waist BW, can be shortened. Since the position of beam waist BW can be varied, symbols can be read with high resolution over a broad range of reading distances.
The complexity of the structural mechanisms necessary to perform the mechanical movement of the laser light source, lenses and other optical components in this first technique causes an increase in manufacturing cost. Additionally, the increase in the number of moving parts leads to lower reliability. Specifically, with reference to the device shown in FIGS. 1A and 1B, when the condenser lens 202 is brought close to semi-conductor laser light source 1 as shown in FIG. 1B, the angle .alpha. subtended at a semiconductor light source 1 by the condenser lens 202 will increase. This increase causes the spot diameter at beam waist BW to also increase. In other words, shown in FIG. 2, the spot diameter at the beam waist BW increases in substantial proportion to the distance FL. As a result, symbols which are a remote distance from the symbol reading apparatus cannot be read with high resolution.
A second prior art technique that also successfully solved the aforementioned problem is described in Unexamined Published Japanese Patent Application No. 133891/1990. The apparatus incorporating this prior art technique has a plurality of beam issuing units. Each beam issuing unit is composed of a semiconductor laser and a lens, and are set to have focal points at different reading distances. The beam issuing units are selectively operated in accordance with a particular reading distance or a selected reading distance so that symbols can be read with high resolution over a broad reading range.
This second prior art technique involves the use of multiple semiconductor laser light sources and lenses which causes the cost of the symbol reading apparatus to increase. Additionally, there is the problem that a complicated optical layout is required to achieve registry among the optical paths of the laser beams issued from the plurality of beam issuing units.
A third prior art technique that has successfully solved the aforementioned problem is shown schematically in FIG. 3. In FIG. 3, a beam issuing unit 211 emits a laser beam 212, which has its optical path switched back by a reflector mirror 213A or 213B. The reflected laser beam 212 is again reflected by a polygonal mirror 215. Polygonal mirror 215 is rotated at a constant speed in the direction of arrow 214, and guides laser beam 212 towards a symbol face 216 carrying a bar code to be read. As the polygonal mirror 215 rotates, the direction in which the laser beam 212 travels will change, scanning the symbol face 216 automatically.
The reflector mirror 213B, which is closer to the beam issuing unit 211, is inserted into or retracted from the optical path of laser beam 212 by a drive mechanism (not shown). When inserted, the laser beam 212 is reflected by the mirror 213B, and when retracted laser beam 212 is reflected by mirror 213A. As a result, the optical path length of laser beam 212 is changed, and the distance from the symbol reading device to the focal position where the beam waist of the laser beam is formed, can be varied between two values. Possible modifications include the addition of reflector mirrors between beam issuing unit 211 and reflector mirror 213A. Alternatively, a single mirror can be displaced between reflector mirror 213A and the position of reflector mirror 213B. This symbol reading device permits the beam waist of the laser beam to be formed at varying positions thereby accomplishing high resolution reading of symbols at various distances from the symbol reading device.
The problem with the third prior art technique is that a driving mechanism is required for driving each reflector mirror positioned between beam issuing unit 211 and the reflector mirror 213A. As a result, the number of moving parts increases as the reading range of the symbol reading device increases which causes both an increase in cost and a decrease in reliability. In the modified symbol reading device using a single reflector mirror displaced continuously along the optical path of laser beam 212 between the beam issuing unit 211 and reflector mirror 213A, the long operating distance of the single reflector mirror impairs response of the system.